Power transmission for motor vehicles



DCC- 9 1952 A. A. lsslsoms ETAL POWER TRANSMISSION EOR MOTOR VEHICLES 5 Sheets-Sheet 1 Filed Abril 7, 195o A. A. ISSIGONIS ET AL lPOWER TRANSMISSION FOR MOTOR VEHICLES Dec. 9', 1952 Filed April '7. 1950 Dec. 9, 1952 A. A. lsslGoNls ET AL POWER TRANSMISSION FOR MOTOR VEHICLES 5 Sheets-Sheet 5 Filed April 7, 1950 F/GAI.

Dec. 9, 1952 v A. A. lsslGoNls ETAL 2,620,679.

' RowER TRANSMISSION RoR Mo'roR VEHICLES Filed April '7, 1950 5 Sheets-Sheet 4 A Homey Dec. 9, 1952 A.A.1ss|GoN|s ETAL POWER TRANSMISSION FOR MOTOR VEHICLES 5 Sheeis-Sheet 5 Filed April 7. 1950 'F/GjQ.

ff/Ga.l

Atiorney Patented Dec. 9, l1952 `UNITED STATES s 2,620,679 ATENT OFFICE PGWER TRANSMISSION FOR MOTOR VEHICLES Alexander A. Issigonis, Oxford, John N. Morris,

Edgbaston, Birmingham, and Peter W. Harrison, Croydon, England, assignors to VMorris Motors Limited, Cowley, England Application April 7, 1950, Serial No. 154,602 In Great Britain April 13, 1949 (Cl. 'i4-472) 16 Claims.

This Yinvention relates `to power transmissions for'motorvehicles, of the kind in which two hydraulically-operated friction clutches control respectively low-gear and high-gear trains, and the low-gear train is provided with a freewheel coupling'which overruns when both the clutches are engaged. Y

According `to the yinvention selective engagement'an'd disengagement of the clutches individually at different engine speeds is effected by hydraulic control means arranged to be influenced by the degree of opening of the engine throttle and :functioning in such a way that, whether the engine is running at part throttle or full throttle, so long as the degreexof throttle openhydraulic control means to be influenced by the l degree of throttle opening, the driver is enabled to exercise some control over the speeds at which an automatic gear-change occurs in either direction.

With a vehicle having a sufliciently high powerto-weight ratio the gearbox need only provide for two forward speeds and a reverse, in which case the present invention enables the transmission to operate entirely automatically so far as gear changing in the forward speeds is concerned, i

andthe usual clutch pedal is not required. However, the invention is equally applicable to a transmission of the semi-automatic type having agearbox which affords more than two forward 'speeds and incorporates an automatic under-H drive mechanism in conjunction with manual gear changing, a clutch pedal then being provided.

Clutch engagement is effected by means of oil or other appropriate liquid supplied at a pressure which is determined by the hydraulic control means and is correlated at all times with the prevailing engine speed, and preferably also with the speed of the vehicle. When the vehicle is stationary, with the engine idling, the hydraulic pressure developed is insufficient to cause engagement of either of the clutches. But the arrangement is such that upon accelerating the engine train. Whenthe engine speed falls suciently, the hydraulic control means operates to cause 2 l disengagement of the high-gear clutch Vand the drive reverts to the low-gear train.

It is arranged for engagement and disengagement of the high-gear clutch to take place progressively, so that gear-changing in either direction can be effected without abrupt change in the relationship between the engine and `vehicle speeds.

Referring now tothe accompanying drawings:

Figure 1 is a schematic illustration (partly in section) of an automatic two-speed transmission in Aacordarme with the invention, for motor Y vehicles;

Figure 2 is a view on the line II--I in Fgure'l;

Figure 3 is a sectional elevation of a hydraulic control unit which is employed in the arrangement illustrated in Figure 1;

Figure 4 is a sectional elevation of a ow-retarding device which is preferably associated with the control unit shown in Figure 3;

Figure 5 illustrates how the control unit shown in Figure 3 may be modified in order to compensate for variation of the viscosity of the liquid employed to operate the hydraulic clutches;

Figure 6 is a sectional elevationof part'of a semi-automatic transmission in accordance with the invention,.for motor vehicles, and shows the preferred form of the clutches which are illustrated diagrammatically in Figure 1;k

Figure 7 is a sectionalelevation of an overriding control device employed with the trans'- mission shown in Figure 6; K n

Figure 8 and 9 are sections on the line A-A in Figure 7, showing the two positions of adjustment of the device; and

Figure 10 is a fragmentary sectional elevation showing how the control unit illustrated in Figure 3 may be modified to enable each clutchto be operated by its own diaphragm.

In the case .of the automatic two-speed transmission represented diagrammatically in its simplest form in Figure 1, the two clutches, indicated generally at l and 2, are accommodated in Va substantially sealed casing 3, forming a selfcontained unit, and have their respective driven plates and 5 splined on co-axial shafts iiy and "i, oneof which is tubular and surrounds the other. The tubular shaft 6 drives a pair of constant 4mesh gears 8 and 9 the latter of which has associated .with it a Yunidirectional coupling IS of known design, such as a roller ratchet device, permitting torque to be transmitted inthe driving, but not in the overrunning, direction; this gearing 8, 9 constituting the low-gear train. The high-gear train is formed by a further pair of constant mesh gears Il and I2, these being driven by the shaft 1.

AIn order to promote smooth take-up of the drive, the driven plates 4 and 5 of the clutches are tted with wetvlinings I3, such ascork finserts running in oil. The driven plate 4 of 'the low-gear clutch I is situated between the pressure-plate I4 of this clutch and the pressureplate I5 of the high-gear clutch 2. These pressure-plates are splined on the casing 3, and are spring-loaded so that the clutches remain disengaged until suicient opposing force is applied to effect engagement. The pressure-plate I5 is acted upon by helical compression springs I6 which normally hold it against an abutment I?, and the arrangement is such that the pressure necessary to cause engagement of the high-gear clutch 2 is considerably greater than that which suces to produce engagement of the low-gear clutch I.

When the engine is running, oil, at a pressure regulated by the hydraulic control means (which will be described later), is delivered through a pipe I8 to one side of an annular diaphragm I3 of synthetic rubber which is co-axial with the Y pressure-plates of the clutches. When the oil pressure reaches a sufficient value, the deflection of the diaphragm I9 displaces the pressure-plate I4 of the low-gear clutch I against its springloading. This clutch then becomes engaged, by reason of the linings I3 of its driven plate 4 being gripped between the respective pressureplates I4 and I5, and the drive is transmitted through the low-gear train 8, B. Upon suflicient increase in the oil pressure applied to the diaphragm I9, the latter is able to deect further and displace the pressure-plate I5 of the highgear clutch 2, against the combined effects of the spring-loading of both pressure-plates, so that both clutches are now engaged and the drive is transmitted through the high-gear train II, I2. When the oil pressure decreases suiciently, the high-gear clutch 2 disengages itself and, subsequently, when the pressure has fallen low enough to permit the diaphragm I9 to return to its undeiiected condition under the assistance of the spring-loading of the pressure-plate I4 of the low-gear clutch I, the latter disengages itself and disconnects the drive. A small leakage 23 is provided from the chamber 2I of the diaphragm to ensure ultimate illing of the entire clutch casing with oil, and also to permit eventual de-aeration of the hydraulic system. The complete lling of the clutch casing with oil prevents the diaphragm I9 from being deflected by pressure due tc centrifugal action upon the contents of its chamber 2|. Near its axis the clutch casing has a breathing aperture 22 through which oil can return to a sump (not shown).

The hydraulic control means includes a positive displacement oil pump 23, driven by gearing 24, 25 from the engine shaft 23, the output of this pump being preferably augmented by a similar oil pump 21 driven at a speed proportional to the instantaneous speed of the vehicle, and a control unit 28 which receives the output of the engine-driven pump 23 (together with that of the vehicle-driven pump 2'! when the latter is employed), and regulates the now cf oil to the diaphragm chamber 2I in accordance with the conditions prevailing at any given time. The pumps 23 and 21 draw oil from the sump through intakes 29 and 30 respectively, and discharge into a common delivery pipe 3|.

Although the additional vehicle-driven oil pump 2T is not essential, it is certainly a veryV desirable adjunct. Not only does it serve to activate the control unit 28, and thereby effect clutch engagement, in the event of the engine .having to be started by towing the vehicle; but :its provision also ensures that if the vehicle is allowed to start from rest and run downhill with the engine stopped or merely idling, an unduly hig-h speed cannot be attained bef-ore the drive is taken up. Without that provision in the 4circumstances envisaged, the vehicle speed might become very high and then a touch on the accelerator pedal would cause sudden engagement of the low-gear clutch. Consequently the engine would suffer very rapid acceleration, with unpleasant results.

The control unit 23, shown in detail in Figure 3, comprises a co-axial pair of cylindrical chambers 32 and 33 which will be designated the primary and secondary chambers respectively, these being in permanent communication with each other through a thin-edged metering oriiice 34 centrally disposed at their junction. The eiective cro-ss-sectional area of the metering oriiice is regulated by .the contour of the external surface of a hollow plunger 35 which exten-ds axially through that orifice. One end of the plunger i-s enlarged .to form a piston 33 which works in the primary chamber 32, and its other end, which is closed, passes in u-id-tight manner through the end wall 3'! of the secondary chamber 33.

The plunger 33 is loaded by a pair of helical compres-sion springs 38 and 33 which are accommodated in the primary chamber 32 and urge the plunger into a position at which the piston 33 lies (as shown) adjacent the junction of the chambers. The spring 38, which is of low rate, acts between the end wall 40 of the primary chamber 32 and .the piston 36; and the other spring 33, which is of high rate, loads a disc valve 4I fitted to the open end of the plunger 35, this spring having an adjustable abutment 42 adjoining the end wall 40 of the primary chamber.

The oil delivered to the control unit 28, by the pipe 3i, enters the primary chamber 32 at its junction with the secondary chamber 33 and proceeds into the latter by way of the metering orifice 33 which, when the engine is idling, is Iable to pass the corresponding flow of oil without sufcient pressure being created in the primary chamber 32 to cause movement of the piston 33. Most of the oil passes through holes 43 in the plunger 35 into its bore, opens the disc valve 4I and finally escapes from the primary chamber 32 through a low-pressure outlet 44 from which it returns to the sump. However, a small flow of oil proceeds from the secondary -chamber 33 through an outlet 'A5 connected by the pipe I8 to the diaphragm chamber 2l (Fig. l) of the clutch unit, by reason of the small leakage 23 provided from the latter chamber, but the spring load on the disc valve 4I when the engine is idling is too small to establish a back-pressure suffi-cient. to effect engagement of the low-gear clutch I. In this connection, if the Ispring 39 which loads the disc valve lll is of constant rate, the oil pressure in the diaphragm chamber 2I will be in linear proportion to the compression of that spring, and therefore vto the movement of the piston 33 and its associated plunger 35.

Although the rate of the disc valve `spring 39 and the contour of vthe external surface of the plunger 35 may be so chosen as to afford Iany desired relationship between the speed of the engine and the axial displacement of the plunger; in the particular example und-er consideration the external surface of that portion 46 of the plunger 35 which adjoins the side of the piston 33 remote from the disc valve 4I is of frustoconical shape and is convergent in the direction Vof its spring-loading.

leading lfrom the metering orifice V34Y vinto the secondary chamber 33. At the smaller end of the Afrusto-conical portion 43 there is an annular groove 4:1, 'and then :a substantially cylindrical portion 48 of approximately the same diameter as `the larger end of the truste-conical por-tion 46. Beyond the substantially cylindrical portion 48 the plunger 35 continues as a cylindrical stem of reduced diameter which passes th-rough the end Wall 3.1 of the lsecondary chamber 33.

When `the engine is idling, the larger end of the frusto-conical portion 4B of .the plunger 35 lies Within the metering orifice 34 and, as already explained, the resulting liow of oil is ineffective to cause'engagement of the low-.gear clutch l. Butupon increasing `the engine speed the ef- `fective cross-sectional larea of the metering orice 34 is Ainadequate to pass the increased flow of oil, with the result that the pressure in the .primary chamber 32 rises sufficiently to initiate movement of the piston 3B against the action As the engine is accelerated, the piston 35 continues to move until it attains a position at which the spring 3S which .loads the disc valve y4| has become compressed `to .such an extent that the oil pressure in the secondary chamber 33 is sufficient `to effect engagement of the low-gear clutch l by deflecting the diaphragm I9 (Fig. 1). Further increase of engine speed causes additional movement of the piston 35, accompanied by an increase of the oil pressure acting on the diaphragm. i3. However, before that lpressure can attain a value at which it deflects the diaphragm sufiiciently to initiate engagement of the high-gear clutch 2, the irusto-conical portion 45 of the plunger 35 completely leaves the .metering oriiice 33. The latter now has its effective cross-sectional area abruptly enlarged by the annular groove l of the plunger, and a substantial increase of engine speed now only produces a relatively small additional movement of the piston and plunger.

When the trailing edge of the annular groove lllk of the plunger approaches the plane of the metering orifice 33, the control unit 28 becomes violently unstable because any further movement of the lpiston 35 in the same direction results in a diminution of the effective cross-sectional area of the metering orifice 34, owing to the presence of the substantially cylindrical portion Y 48 of the plunger 35. AConsequently the oil pressure acting on the piston 33 increasesl sharply, and the plunger is displaced suddenly to an extent equal to the axial length Vof its substantially cylindrical portion ll. placement gives rise yto an abruptY increase in the back-pressure exerted by the spring 39 which acts on the disc valve M, and, in this way, at a predetermined engine speed, the oil pressure applied to the diaphragm I9 rises steeply to a value at which the high-gear clutch 2 is iirmly engaged. This condition persists notwithstanding that the engine speed has fallen considerably .due tothe engagement of the high-gear clutch, and the speed of the vehicle has not altered appreciably in the process. The maintenance of the considerable displacement of the plunger 35 can be assured by making its substantially cylindrical vportion v48 .of sufficiently large diameter with respect to that 'of the metering orifice 34. For example, if the portion 48 of the plunger actually fitted the metering orifice the plunger would not revert to its initial position until the engine stQpped. I n practice, thediameter of .the

portieri v43 visso chosen that, with the high-gear ThisI disclutch 2 engaged, the yoil pressure maintained is insuiiicient to keep that clutch 'in engagement when the speed 'of the vehicle has fallen to a given value. Clearly this value should be sufciently below that at which, with the .low-gear clutch I alone engaged, the upward change of gear is arranged to occur. Forv example, if the automatic change from low to high gear takes place when the vehicle attains a speed of about 30 M. P. H., matters would be so 'arranged that the change from high to low gear would not occur until the speed had dropped to about 2O M. P. I-I.

The operation of the hydraulic control unit 231s influenced by the degree of throttle vopening.v This is achieved by providing across the metering oriiice 33 a by-pass |49 controlled by a progressively opening valve 53 which is actuated by a linkage 5l coupled with the accelerator pedal which pedal controls the throttle. The arrangement is such that when the throttle is fully opened ytheby-pass 49 is fully eiiective', land it is then necessary for the engine to attain a relatively high speed in low gear before the change-up will occur. Similarly, with the b-ypass 49 open, the change-down will occur whilst the speed of the vehicle is still relatively high. With the -by-pass almost closed, however, a relatively low engine speed will suiiice to cause the change-up, but the change-down will not take place until the speed of the vehicle has dropped to a relatively low value. It will be appreciated that the particular engine and vehicle speeds at which clutch engagement commences,v and at which gear-changing occurs in either direction, are adjustable by variation of the shape and disposition of the contoured portions of the plunger 35, and also by varying the loading and rat-ing of the two springs 38 and 39.

As already indicated, it is arranged for engagement and disengagement of the high-gear clutch -2 to take place gradually in order to avoid any sense of shock or sudden discontinuity in the relationship between the engine and vehicle speeds at the moment of gear-changing. Such gradual engagement could be achieved by providing sufiicient breathing capacity in the diaphragm chamber 2l of the clutch unit, or by the inclusion of an air bottle or its equivalent in the oil line i8 to the clutch unit. However, We prefer to incorporate in that oil line a now-retarder (Fig. 4) comprising a vertically disposed cylindrical chamber 52 containing a hollow piston 53 which is sandwiched between a pair of centralising springs 54. The upper face of the piston has a central thin-edged aperture 55 which is largely obstructed by an axially disposed restrictor 56, the latter cbeing essentially of synclastically convex shape and rigidly supported from the top of the chamber. The arrangement is such that normally the plane of the aperture 55 lies at the level of the maximum 'bulge of the restrictor 5S, and consequently the effective area of the aperture 55 is then the minimum capable of permitting the small flow of oil that continually takes place, when the engine is running, due to the leakage 23 provided from the diaphragm chamber 2l of the clutch unit.

The oil coming from the outlet l5 of the secondary chamber 33 of the control unit 23 is delivered to an inlet 5i at Athe top of the flowretarder chamber 52, passes through the yrestricted aperture55 vinto the interior of the piston 53, and then through holes 58 in the wall of `Athe `,piston to ,a pipe 59 which leads .from the side of the flow-retarder chamber to the diaphragm chamber 2l of the clutch unit by way of the pipe I8. The piston 53 of the flow-retarder remains in its normal central position whether the vehicle is running in low or high gear. But change of gear either up or down, although primarily initiated by an alteration in the static pressure only of the oil delivered by the control unit 2S, must nevertheless be accompanied by some ilow in one direction or the other by reason of the breathing capacity (limited though it may be) of the diaphragm chamber 2l of the clutch unit. Such a flow is not immediately possible to the extent which would ordinarily be necessary, because the elective area of the aperture i5 associated with the restrictor is at its minimum. In consequence, upon change of gear being initiated in either direction a substantial difference of pressure is established across the aperture 55 and causes the piston 53 to move, with the result that the eiective area of that aperture becomes greater temporarily to cater for the transient increased iiow. The lower end of the 'piston 53 is immersed in oil 5S and has a small hole El in it to provide a dash-pot action for retarding the movement of the piston, thus preventing a too rapid extension or deflation of the diaphragm i9.

In cold weather, the viscosity oi the oil in the hydraulic circuit may be suiciently high to occasion premature engagement of the low-gear clutch I at starting, owing to the increased viscosity of the oil creating a suiicient difference of pressure across the metering orifice 3d of the control unit 2S to cause premature movement of the plunger 35. In order to compensate for the eiect of variation of the oil viscosity upon the initial engagement oi the low-gear clutch i, the control unit 2S is modified as indicated in Figure 5. The pipe 3l through which oil is pumped to the primary chamber 32 of the control unit 2E is provided with a long narrow section or similar tortuous passage S2 which offers high resistance to the flow of abnormally viscous oil. When the oil is of normal viscosity it can fdow through the passage e2 and into the primary chamber oi the control unit during the initial process of lowgear clutch engagement without building up a substantial pressure diierence. If, however, the oil is suiiciently cold the pressure engendered by this path causes a considerable back-pressure; that is to say, a substantial build-up in pressure on the inlet side of the restrictive passage 52. A connection C3 leads from this point to an auxiliary chamber which is preferably co-axial with the secondary chamber 33 oi the control unit 23 and contains a compensating piston E5 attached to the stem of the plunger 35 of that unit. The auxiliary chamber has a vent 66 to atmosphere and the arrangement is such that, when the oil viscosity is abnormally high, the increased bachpressure occurring at the inlet side o the restrictive passage t2 is transmitted to the compeneating piston $5 in such a way that this piston, and thus the plunger 35 oi the control unit, is abnormally loaded in a direction which tends to prevent its premature movement which would otherwise result from the abnormally viscous condition of the oil.

in unduly high pressures arising at the inlet to the restrictive passage 62 when the oil iiow becomes appreciable with further increase in engine speed above that obtaining during the initial clutching stage, a springloaded by-pass valve El is arranged, in an enlargement SIA of the pipe 3l, in parallel with 8 the restrictive passage 62. This valve opens when the pressure build-up across the restrictive `passage 62 exceeds a certain value. The arrangement is such that when the engine speed is high, and thus the volume of oil iiow is considerable, the by-pass valve 51 will normally be maintained open. Under these conditions the arrangement merely imposes a finite and limited back-pressure which is always operative upon the compensating piston E5 under conditions of high engine speed.

Reverting to Figure 1, the nal drive to either the front or rear wheels of the vehicle comprises the usual bevel pinion 68 and crown wheel 69. The pinion is slidable on its splined shafts 5, by a manually-operable gear selector 1I), into the positions indicated at 8A and 8B. When the pinion il is at 8A the transmission is in neutral, and when it is at 8B a reverse gear train 1I, 'I2 is engaged. A spring-loaded ball-valve 'I3 is arranged, as shown, in communication with the oil inlet pipe I8, this valve controlling a pressure-relief pipe i4 through which oil can be spilled to the sump. So long as the pinion ti is in engagement with the gear the shaft 'l5 of the gear selector l' depresses a control plunger 'i6 which compresses the spring of the ball-valve 'I3 suiiiciently to permit the eventual development oi an oil pressure adequate to eifect engagement of the highgear clutch 2 after engagement of the low-gear clutch l. However, selection of either neutral or the reverse gear train causes the selector shaft i5 to recede from the oil-spill control plunger 15, and the consequent reduction of the springloading on the ball-valve I3 results in the latter relieving the oil pressure by Way o the pipe 14, so that the available pressure is suiilcient only to operate the low-gear clutch l.

Figure 6 shows the preferred form of construction of the clutch unit (which, as already pointed out, is only represented diagrammatically in Figure l), and illustrates part of a semi-automatic transmission in accordance with the invention. Corresponding parts in Figures l and 6 are designated by the same reference numerals, but the two oil pumps 23 and 2l and the control unit 2t are not shown in Figure 6. The abutment I'I (Fig. 1) for the pressure-plate I 5 of the high-gear clutch is constituted by separate members, none of which appear in the particular cross-section represented by Figure 6. The pressure-plate l5 is mounted on three blade springs 'I'I disposed as symmetrical chords and secured to a iixed plate 18, only a portion of one of these blade springs being visible in Figure 6. The same arrangement of blade springs is also employed for resiliently mounting the pressure-plate I4 on a iixed ringi9, but none of these springs is visible in Figure 6. The pressure-plate I4 is acted upon by tension springs 8d which assist the return of the diaphragm I9 to its undelected position upon disengagement ci the low-gear clutch I. The diaphragm is bears against a ring ,GI which is carried by the pressure-plate i4. The restricted leak 2S (Fig. l) from the diaphragm chamber 2I is constituted, in the construction shown in Figure 6., by ducts 82 and a clearance space B3 around the shaft t. A duct Bil conveys the oil from the inlet ifi to the diaphragm chamber 2i.

The low-gear train 8, 9 and the high-gear train II, I2 in the case of the transmission illustrated in Figure 6 are accommodated in the front portion of a manually-controlled gearbox 35 of orthodox design affording three or four forward speeds and a reverse drive. This gearbox has .a mainshaft 88 and arlaysh'aft 81, the latter carrying' the gears 9 and l2 and the unidirectionalcoupling l0.- Itwill be appreciated that the automatic' operation of the clutches i and- Z takes' place in the manner already described with reference to Figure 1', whichever of the forward' speeds of the gearbox 85 is in use at any time. That is tofsay, an underdrive through the-low-gear train- 8,9y isY associated with each of the forward gear ratios.

InV order'to permit 4manual gear changing in the gearbox 85, provision must be made whereby the driver can override the control unit 28 and effect disengagement of both-theclutches-I and?. All that isnecessary in this respect is to include, in the delivery pipe I8v (Fig. 6) leading to the diaphragmchamber 2l, some form of control valve operable by a pedal which corresponds with the normal clutch pedal. One form of overriding control device 88 is' illustrated in Figures 7 to 9. It comprises av two-way rotary plug valve y39 which is-v fittedin the pipe I8A and isfactuated by a: lever 98 linked to they clutch pedal (not shown). The valve-89`is formed with an inclined notch 9| 'and the arrangement is such thaty when thisvalve is in the setting indicated in Figures 7 and 9vthe oil delivered rtothe pipe I 8 from the-control unit 28, instead oi-N proceeding to the diaphragm chamber 2l, is divereted to the sump through a by-pass pipe-92". At the'same time, the pressure o'f the Voil"within the diaphragm chamber y2l is relieved bywayr of a pipe V93 which leads to the sump'.V When, however, the valve 39 is rotated into the' Vposition shown iny Figure 8, the oil deliveredl from the control unit 28 passes into the diaphragm chamber 2l to effect clutch engagement.

yInstead of employing a'single diaphragm i9 for operating both of the clutches I and 2 in the manner described, each of these clutches may be actuated by its own diaphragm. In this case the control unit 23 isjmodied as shown in Figure 10. The disc valve4-i is nowloaded by a spring' 94' of constant rate, instead of by the high-rate spring 39V (Fig. 3), and the hollow plunger A351s lengthened and provided withvtwo further sets ofv holes 95 and 96; chamber33 is delivered by way of the holes 4'3 and 95 to'ra port'91'and thenceto a duct 93 which is connected to the diaphragm chamber of the low-gear clutch l. When the piston 36 rises sufficiently, owing tov increase of the oil pressure' in the manner previously explained, the holes 96 comer into communicationiwith a port 99 having associated with'it a duct' Ill. The latter is connected to the diaphragm chamber of the highgear clutch 2, the'flow-retar'der 52 (Fig. 4) being interposed, so that this clutch then becomes engaged.

When employing separate diaphragms for operating the clutches of the semi-automatic transmission (Fig. 6), it is of course necessary to provide an overriding control'device (such as that illustratedin Figures 7 to 9) in each of the pipes which'connect the ducts Sil-and |00 (Fig. 10)'with the respective diaphragm chambers, thel two overriding control devices being interlinked so as to be operablein unisonby the clutch pedal.

We claim:

1. A power transmission for motor vehicles including a system forengaging and disengaging a clutch, having a pressure source, clutch actuatingmeans, a fluid` circulating therebetween, a control unitV located between said'pressure source and said clutchV actuating means, said control unit comprising a passageway for said fluid having an Oil from the secondaryV 10 inlet, an outlet, an intermediate metering orifice" and a variable pressure-relief valveon the outlet side of said metering orifice, the pressure re'- quired to open said relief valve being responsive to the pressure in said inlet, a` movable control member at said metering orifice responsive to'the pressure in said inlet,rsaid control member vary?"- ing as it moves the effectivel areafof the metering orifice and thus regulating the pressure in the inlet and consequently the pressure required to: open the relief valve and the pressure at the outlet, said control member lopening saidoric'e as it moves in response to increased inlet pressure suddenly to a greater, then suddenlyV toa lessand finally to a. constant portion ofits total area,said sudden decrease in area terminating asfthe inlet` pressure equals that required` to operate the clutch actuating means, as a result of which said control member is then subjected tosudden prese sure and forced well beyond the oriii'ce" varyingportion thereof so thatmoderate'subsequent'del creases in inlet pressure andl consequent move-'- ment of the control member' have no substantial effect on the effective area ofY the orifice and existing pressure against the clutch actuating means.

2. A power transmission as claimed in claim 1 including also an engine shaft, constant mesh low-gear and high-gear trains having separate drive shafts, a pair of friction' clutchesl each disposed between the engine shaft and the'corresponding one of said drive shafts,l an output' shaft common to said gear trains, andv a frees' wheel coupling associated with the low-gear train, said coupling overrunning when s'aid clutches are both engaged, andthe pressure supplied by said pressure source being responsive to change in the speed of the engine shaft for effecting selective engagement and disengagement of said clutches individually atdiiferent engine speeds.

3. A power transmission as clai'medin claimjZ in which said pressuresource comprises a positive displacement pump driven' byr said engine which a channel bypasses the metering orifice andV flow in the channel isregulated by a'valve which moves with movement of lthe engine throttle.V

7. A transmission as claimed in claim 1 in vwhich the control unit passageway extends 'on' the inlet side of the metering orifice to an end--I wall further from the orifice than the inlet and in which ythe control member is aspring-loade'd' plunger extending through said metering orifice"L and enlarged on the inlet side thereof to form a piston working in said passageway beyond tl'i'eVv inlet.

8. YA transmission as claimed in claim 7 which said plunger is hollow andcarrie's the" pressure relief valve at the end nearerthe control unit inlet.

9. A variable-speed power transmission for a motor vehicle in which two hydraulically-operated friction clutches control respectively low-gear and high-gear trains and the low gear train is provided with a free-whee1 coupling which overruns when both the clutches are engaged, and in which a flow of pressure-liquid effects selective engagement and disengagement of the clutches individually at diierent engine speeds, said ilow being regulated in accordance with the conditions prevailing at any one time by a hydraulic control unit which receives the combined outputs from two positive displacement pumps driven one by the engine and the other at a speed proportional to the instantaneous speed of the vehicle, said hydraulic control unit comprising a coeaxial pair of primary and secondary cylindrical chambers in permanent communication with each other by means of a thin-edged metering orifice through which the pressure liquid flows from the primary to the secondary chamber before being discharged from the unit, and a spring-loaded plunger slidable in said oriiice which plunger is enlarged at one end to form a piston working in the primary chamber, the external diameter of said plunger varying to regulate the eiective cross-sectional area of the metering orifice upon axial displacement ci the plunger in response to variation of the liquid pressure acting upon the piston, the action of the control unit being such that the automatic change from the high-gear to the low-gear train cannot occur until the vehicle speed has fallen considerably below that at which the high-gear clutch became engaged.

10. A transmission as claimed in claim 9 for use in association with an engine having a throttle and control means therefor, in which the flow of pressure liquid through said control unit also regulated by movement of a valve responsive to movement of the throttle control means.

11. A variable speed power transmission for motor vehicles according to claim 10, in which the plunger is hollow and is provided with holes which enable the pressure-liquid to enter its bore from the secondary chamber, communication between the bore of the plunger and the primary chamber being regulated by a springloaded disc valve which, when open, permits spillage of pressure liquid from the primary chamber.

12. A variable-speed power transmission for motor vehicles according to claim 9, in which the clutches are accommodated in a substantially sealed casing containing an annular iiexible diaphragm which, when subjected to the action of Y liquid pressure regulated by the hydraulic control unit, is deiiected first to an extent suilcient to cause engagement of the low-gear clutch alone and then to an extent suioientV to eiTect engagement of the high-gear clutch upon the applied liquid pressure attaining a sufficiently high value.

13. A variable-speed power transmission for a motor vehicle in which two hydraulically operated friction clutches control respectively low-gear and high-gear trains, and the low-gear train is provided with a free-wheel coupling which overruns when both the clutches are engaged, and in which a ow of pressure liquid effects selective engagement and disengagement of the clutches individually at different engine speeds, said flow being regulated in accordance with the conditions prevailing at any one time by a hydraulic control unit which unit comprises a co-axial pair of primary and secondary cylindrical chambers in permanent communication with each other by means of aV thin-edged metering oriiice through which the pressure liquid flows from the primary to the secondary chamber before being discharged from the unit, and a spring-loaded plunger slidable in said orice which plunger is enlarged at one end to form a piston working in the primary chamber, the external diameter of said plunger varying to regulate the effective cross-sectional area of the metering orificerupon axial displacement of the plunger in response to variation of the liquid pressure on the piston, the diameter of one longitudinal Zone of said plunger increasing sharply to that of a zone of constant diameter, the plunger being so positioned relative to said orice that as the pressure on said piston approaches that required to engage the high-gear clutch it forces the zone of sharply increasing diameter through the orifice, thereby sharply reducing the effective cross-sectional area of the orifice and suddenly increasing the pressure on the piston, thus forcing the Zone of constant diameter well into the orifice so that the piston pressure must be reduced suiiiciently to draw this zone of constant diameter out of the orice before the high-gear clutch will be disengaged.

14. A transmission as claimed in claim 13 for use in association with an engine having a throttle and ccntrol means therefor, in which the flow of pressure liquid through said control unit is also regulated by movement of a valve responsive to movement of the throttle control means.

15. A variable speed power transmission for motor vehicles according to claim 14, in which the plunger is hollow and is provided with holes which enable the pressure-liquid to enter its bore from the secondary chamber, communication between the bore of the plunger and the primary chamber being regulated by a spring-loaded disc valve which, when open, permits spillage of pressure-liquid from the primary chamber.

16. A variable-speed power transmission for motor vehicles according to claim 13, in which the clutches are accommodated in a substantially sealed casing containing an annular flexible diaphragm which, when subjected to the action of4 liquid pressure regulated by the hydraulic control unit, is deflected first to an extent suiicient to cause engagement of the low-gear clutch alone and then to an extent sufcient to eifect engagement of the high-gear clutch upon the applied liquid pressure attaining a suiciently high value.

ALEX. A. ISSIGONIS. JOHN N. MORRIS. PETER W. HARRISON.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 794,899 Sturtevant July 18, 1905 876,164 Ford Jan. 7, 1908 1,523,648 Jackson Jan. 20, 1925 1,877,102 Whitesell Sept. 13, 1932 2,094,449 Forichon Sept. 28, 1937 2,159,170 Maybach May 23, 1939 2,272,684 Vickers Feb. 10, 1942 2,335,829 McBride Nov. 30, 1943 FOREIGN PATENTS Number Country Date 151,203 Austria Oct. 25, 1937 729,567 France July 26, 1932 890,868 France Feb. 2, 1944 

